Some people list scrap wood on Craigslist. Another option is construction sites, but you should always ask the construction managers.
habitat and resource type location are also good places. I second craig's list. When I was building my cabin I got some great deals on some windows...brand new wood clad window...for 20 buck...this would have been 200 if ordered new.
Hi: I get pallets from a furniture store near me at times he has many of them, also try ware houses and stores like home depot or Walmart, ect.